The development, in the 1970's, of economic processes for fabricating low loss optical fibers opened the field of optical communications which has had a major impact on information management and movement. Initial efforts were concentrated in the transmission area. In these initial efforts, optical fiber was used as a high bandwidth transmission medium that was interposed in an otherwise electronic matrix. Accordingly, electrical signals representative of intelligence were transformed into optical signals for transmission and then, at the receiving end, were retransformed back into electrical signals for further processing. However, it was always realized that maximum economies would be obtained if the signals could also be processed in optical form, instead of simply being transmitted in optical form. Accordingly, techniques for optical switching and computing continue to be developed. However, among the first areas into which optical communications has branched, beyond transmission, is the field of optical amplification.
Heretofore, amplification and regeneration required transformation of the optical signal into electrical form. However, recently commercially viable optical amplifiers have been fabricated, primarily using erbium-doped optical amplifiers. Such amplifying fibers exhibit low noise, relatively large, polarization-independent bandwidth, reduced cross-talk, and low insertion loss, and are relatively cheap to fabricate. The fibers can be coupled end-to-end to a transmission fiber and transversely coupled through a directional coupler to a laser diode pump. However, the gain could exhibit undesirable fluctuations due to saturation effects. Recently, electro-optical feedback loops have been used to reduce this gain fluctuation (E. Desurvire et al, CLEO91, Baltimore, Md., April 1991, Paper CThJ3). Additionally, further improved gain stabilization schemes have been reported using all optical feedback loops (M. Zirngibl, Electronics Letters, Vol. 27, No. 7, pg. 560, March 28, 1991). However, due to the very long lifetimes of the relevant excited states that partake in the amplification, fast gain switching which would be needed, for example, in packet switching, appears to be unattainable. Nevertheless, to direct traffic flow, slow gain switching schemes may still be of commercial interest.